This application claims benefit of Japanese Application No. 2001-324970 filed in Japan on Oct. 23, 2001, the contents of which are incorporated by this reference.
The present invention relates to variable resistance circuit implemented on an integrated circuit and having an equivalent resistance between terminals capable of being controlled (regulated) by an electrical signal to be applied to a control terminal, and also relates to application circuits using the variable resistance circuit such as an integrating (filter) circuit, gain control amplifier, automatic gain control circuit.
Among known filter circuits capable of regulating passband (Q-factor) are:
{circle around (1)} Switched Capacitor type filter;
{circle around (2)} Operational Transconductance Amplifier (OTA)-C type filter; and
{circle around (3)} MOSFET-C type filter.
These filters can be implemented on an integrated circuit.
Here, the passband or Q-factor in each filter is regulated (controlled) by:
{circle around (1)} clock (frequency) in Switched Capacitor filter;
{circle around (2)} regulating voltage or current of OTA in OTA-C filter; or
{circle around (3)} the gate voltage of MOSFET in MOSFET-C filter.
Shown in FIGS. 1A, 1B are the configurations of a single-end MOSFET-C type filter and totally balanced MOSFET-C type filter as disclosed in FIGS. 7 and 2 of U.S. Pat. No. 4,509,019. The passband fc thereof and resistance Rds between the source and drain of MOS transistor are given by formulas (1), (2). Included in FIGS. 1A, 1B are: operational amplifier 501; capacitance 502; MOS transistor 503; control voltage source 504; totally balanced operational amplifier 511; capacitances 512n, 512p; MOS transistors 513n, 513p; control voltage source 514; and reference voltage source 515.
fc=1/(2xcfx80xc3x97Cxc3x97Rds)xe2x80x83xe2x80x83(1) 
Rds=1/{xcexcnxc3x97Coxxc3x97W/Lxc3x97(Vxxe2x88x92Vth)}xe2x80x83xe2x80x83(2) 
xcexcn: mobility of electron
Cox: capacitance of gate oxide film of MOS transistor
W: gate width of MOS transistor
L: gate length of MOS transistor
Vth: threshold voltage of MOS transistor The above formulas (1), (2) indicate that MOSFET-C type filter is an integrating circuit having resistance Rds formed between the drain and source of MOS transistor and controlled by voltage Vx to be applied between the gate and source and time constant achieved by capacitance C. The above formulas (1), (2) hold only when the MOS transistor is caused operate in a triode region. FIG. 2 superimposes the triode region over Vgs-Ids characteristic of MOS transistor. FIGS. 3A and 3B show the triode region in an enlarged manner and a type of the drain-to-source resistance.
The drain-to-source resistance Rds of MOS transistor is a function of threshold voltage Vth, and such value is changed by ambient temperatures and variance in the manufacture of MOS transistor. For this reason, the passband of MOSFET-C type filter to be determined by the combination of the drain-to-source resistance Rds and capacitance C is also changed by the ambient temperatures and the variance in the manufacture of MOS transistor. Such change exceeds xc2x150% of a set target value. Because of such characteristic, the filters of the conventional configuration shown in FIGS. 1A, 1B cannot be used in those applications where a high accuracy in passband is required.
Shown in FIG. 4 is the configuration of a totally balanced Tow/Thomas type Biquad filter using the drain-to-source resistance of MOS transistor based on a similar operation principle (disclosed in (1) J. Tow, xe2x80x9cActive RC Filtersxe2x80x94a Statexe2x80x94space Realization,xe2x80x9d Proc. IEEE, Vol. 56 pp1137-1139, 1968, (2) L. C. Thomas, xe2x80x9cThe Biquad: Part Ixe2x80x94some Practical Design Consideration,xe2x80x9d IEEE Trans. Circuits and Syst., vol. CAS-18, pp 350-357, 1971, (3) T. C. Thomas, xe2x80x9cBiquad: Part II:xe2x80x94A Multipurpose Active Filtering System,xe2x80x9d IEEE Trans. Circuits and Syst., Vol. CAS-18 pp. 358-361, 1971.).
In this filter, of the resistances in FIG. 5 for showing the principle of the biquad filter, the resistances Ra, Rb, Rc, Rd, except Rr to be used in producing inverting signal are replaced by the drain-to-source resistance Rdsan, Rdsap, Rdsbn, Rdsbp, Rdscn, Rdscp, Rdsdn, Rdsdp of MOS transistors Man, Map, Mbn, Mbp, Mcn, Mcp, Mdn, Mdp. The passband fc and Q-factor are expressed by formulas (3) to (11). It is to be noted that FIG. 4 includes the totally balanced operational amplifiers 521, 522; capacitance Can, Cap, Cbn, Cbp; control voltage source 523; and reference voltage source 524.
fc=1/(2xcfx80xc3x97Rdsbxc3x97Rdscxc3x97Caxc3x97Cb)xe2x80x83xe2x80x83(3) 
Q={square root over ( )}(Ca/Cb)xc3x97{Rdsb2/(Rdscxc3x97Rdsd)}xe2x80x83xe2x80x83(4) 
Rdsx=1/{xcexcnxc3x97Coxxc3x97W/Lxc3x97(Vxxe2x88x92Vth}xe2x80x83xe2x80x83(5) 
x: a, b, c, d (n, p)
Rdsap=Rdsan=Rdsaxe2x80x83xe2x80x83(6) 
xe2x80x83Rdsbp=Rdsbnxe2x88x92Rdsbxe2x80x83xe2x80x83(7)
Rdscp=Rdscn=Rdscxe2x80x83xe2x80x83(8) 
Rdsdp=Rdsdn=Rdsdxe2x80x83xe2x80x83(9) 
Can=Cap=Caxe2x80x83xe2x80x83(10) 
Cbp=Cbn=Cbxe2x80x83xe2x80x83(11) 
Like formulas (1), (2), the formulas (3) to (11) hold when all MOS transistors are caused to operate in the triode region. Formula (4) indicates that, unlike the passband given by formulas (1) and (2), Q of Biquad filter is determined by the ratio of capacitance and the ratio of drain-to-source resistance Rds between the plurality of MOS transistors. If this filter is used with setting a high Q-factor such as  greater than 4, the signal processing characteristic is affected by the capacitance obtained at an integrated circuit or by the variance to be determined by the ratio of the drain-to-source resistances of the plurality of MOS transistors.
As has been described above, MOSFET-C type filter shown in FIGS. 1A, 1B or in FIG. 4 has restrictions as follows:
{circle around (1)} MOS transistors within the circuit must be operated in the triode region.
{circle around (2)} The passband is shifted xc2x150% or more from the standard value due to ambient temperature and variance in the manufacture of MOS transistor.
{circle around (3)} Although Q-factor, when compared with the passband, is not likely to be affected by variance in the manufacture of transistor and temperatures, a small change in its value affects signal processing in a setting of Q greater than 4.
A conventional automatic gain control circuits (AGC) will now be described. The automatic gain control circuits are used in various circuits including receiving circuits of communication equipment which require amplification of wide dynamic range signals, read circuit of magnetic or optical disk device, code reader devices, oscillation circuits, etc.
Shown in FIG. 6 is the configuration of automatic gain control circuit shown as an example in Japanese patent laid-open application Hei-6-208644. This automatic gain control circuit includes: amplifier circuit 552 including gain control means 551; feedback circuit 553; subtraction circuit 554; and instruction signal 555. The gain A thereof is determined as in formulas (12) and (13) by the internal circuits {operational amplifier 551a, MOS transistor 551b, resistors 551c (R1) and 551d (R2)} of the gain control means 551.
A=R2/R1xc3x97{1/(1+R2/Rds)}xe2x80x83xe2x80x83(12) 
Rds=1/{xcexcnxc3x97Coxxc3x97W/Lxc3x97(Vgsxe2x88x92Vth)xe2x80x83xe2x80x83(13) 
xcexcn: mobility of electron
Cox: capacitance of gate oxide film of MOS transistor
W: gate width of MOS transistor
L: gate length of MOS transistor
Vgs: gate-to-source voltage of MOS transistor
Vth: threshold voltage of MOS transistor
The output voltage of the automatic gain control circuit (in stable state) becomes constant if the gain at the amplifier means 552 and at the feedback means 553 result in a sufficiently large negative-feedback loop. The gain of the system is determined by the characteristic of MOS transistor as indicated by formulas (12) and (13). For example, Vth (standard value 0.8V) varies xc2x10.1V or more due to variance in the manufacture. Such change becomes a cause of fluctuation in rise time and input voltage range.
It is an object of the present invention to provide a variable resistance circuit (the drain-to-source resistance of MOS transistor) capable of offsetting the variance in the manufacture of integrated circuits and the influence of ambient temperatures, and also to provide application circuits using the variable resistance circuit such as an integrating circuit, filter circuit, gain control circuit, and automatic gain control circuit.
To achieve the above object, there is provided a variable resistance circuit in accordance with the present invention, including: a control circuit having a plurality of constant-current output terminals, with a constant-current output ratio of the output terminals varied by electrical signals to be applied to a control terminal; an operational amplifier; a resistor; and a plurality of MOS transistors having the gates thereof connected in common. One output terminal of the control circuit is connected to one end of the resistor and to an inverted input terminal of the operational amplifier; the other output terminal of the control circuit is connected to the drain of one MOS transistor connected at the gate thereof to an output terminal of the operational amplifier and is connected to a non-inverted input terminal of the operational amplifier; the other end of the resistor and the source of said one MOS transistor are connected to a reference voltage terminal; said one MOS transistor is caused to operate in a triode region so as to use as an equivalent resistance the portion between the source and drain of the other MOS transistor which is connected at the gate thereof to the output terminal of the operational amplifier and at the source thereof directly or through a voltage copy circuit to the reference voltage terminal.
In thus constructed variable resistance circuit, the same voltage is applied between the respective gates and sources of the plurality of MOS transistors. This voltage is obtained in such a controlled state as to make equal to each other the drain-to-source resistance of one MOS transistor which serves as a reference and the resistance value (Re) of the resistor by the output current of the control circuit and by the operational amplifier. The portion between the drain and source of the other MOS transistor to which the voltage is applied between the gate and source becomes an equivalent resistance having a value Re/n (n: a positive constant determined by the control circuit).
Further, in the variable resistance circuit according to the invention, the control circuit can be controlled by digital signals to regulate the resistance value. Also, the resistor of the variable resistance circuit can be formed as a discrete resistor so that it is possible to implement the variable resistance circuit having the same temperature characteristic as such discrete resistor.
Furthermore, the variable resistance circuit according to the invention can be used to form an integrating circuit or filter circuit by implementing a voltage copy circuit between the inverted input terminal and non-inverted input terminal of an operational amplifier to which capacitor is connected between the output terminal and the inverted input terminal.
Further, a plurality of time constants are achieved by constructing the variable resistance circuit according to the invention as capable of obtaining a plurality of equivalent resistance and at the same time by providing the same number of units of operational amplifier and capacitor as the equivalent resistances. These can be combined to form a high-order filter circuit.
Further, the variable resistance circuit according to the invention can be constructed so as to obtain two equivalent resistances and these can be combined with a totally balanced operational amplifier and two capacitors to form a totally balanced integrating circuit or totally balanced filter.
Further, an inverting amplifier circuit consisting of resistor and operational amplifier can be combined with the variable resistance circuit according to the invention to form a gain control amplifier.
Further, a controlled system and feedback element can be combined with a control apparatus having a gain control amplifier formed by using the variable resistance circuit according to the invention as described above to construct an automatic gain control circuit.
Further, of the automatic gain control circuit, the controlled system can be a resonance motor which includes: a permanent magnet for providing a magnetic field; a scan mirror cyclically oscillated to reciprocatingly scan a laser beam in predetermined direction; an elastic member for supporting the scan mirror and for providing an oscillating drive; and a coil having two windings consisting of driving winding and detecting winding disposed adjacent to the permanent magnet.
Furthermore, the variable resistance circuit according to the invention can be used to achieve a photoelectric current detecting circuit such that a voltage copy circuit is constructed by two inverting amplifiers of identical construction, a constant-current source connected to the input of one of the inverting amplifiers, and resistor connected between the input and output of said one inverting amplifier and that one terminal of the variable resistance circuit is connected to the output terminal of the other inverting amplifier and that the other terminal of the variable resistance circuit and a light detecting device are connected to the input terminal of the other inverting amplifier.